[TASK] Deprecate DocumentTemplate->extJScode property

The public property DocumentTemplate->extJScode can be used to fill in
ExtJS-specific code to be loaded on "onExtReady". The functionality is not
used in the core anymore, as there are better ways to include JavaScript
code in a backend response.

Resolves: #80512
Releases: master
Change-Id: I4cf24f6b096a79d76e1d30f587f25dcc8ca10fa4
Reviewed-on: https://review.typo3.org/52210


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>